Conventional edge-emitting semiconductor devices such as lasers and semiconductor optical amplifiers (SOA""s) usually comprise a substrate, on which epitaxial layers of varying alloy compositions, carrier types and carrier densities have grown. These various layers are used to define the optical waveguide and the gain region of the device, which are designed to support amplification and emission of radiation in a single spatial mode. Typically, for high power devices, the dimensions of the cross-sectional area of the single spatial mode are a few micrometers in the direction parallel to the epitaxial layers (the lateral direction) and a fraction of a micrometer perpendicular to those layers (the transverse direction).
For many applications, it would be useful if the dimensions of the mode could be made larger in both the lateral and transverse directions, and particularly in the transverse direction. Larger dimensions would lead to a reduction in the numerical aperture of the output beam, and the beam could be more nearly round, rather than being elliptical in cross-section with a large aspect ratio of the major to minor axes. These larger mode sizes and shapes would be especially attractive when the optical output of the semiconductor device is to be coupled into a single-mode optical fiber.
The disjunction between the mode size of a typical semiconductor laser and the mode size in the optical fiber necessitates coupling optics, such as discrete lenses and fiber lenses, to expand the mode when coupling from a semiconductor laser to fiber, or shrink the mode when coupling from fiber to the semiconductor laser or SOA.
Fundamentally, the achievement of a larger mode size in a waveguide that supports only a lowest-order, single spatial mode is a design dilemma. For a symmetric 2-dimensional slab wave-guide, single-spatial-mode propagation can occur only if the following inequality is satisfied:
(xcex94n)n less than 0.5(xcex/W)2,xe2x80x83xe2x80x83Eq. 1.1
where n is the index of the slab, W is its width, xcex is the free-space wavelength, and xcex94n is the difference between the slab index and its cladding index.
If the mode dimension W is on the order of 5 to 10 micrometers, as is found in common single mode fiber, then xcex94n (assuming that xcexxcx9c1 micrometer and nxcx9c3.5) must be less than about 0.006. Such small index differences are nearly impossible to obtain accurately by choosing different alloy compositions, because variations in temperature, carrier density, or gain and loss can easily cause larger changes in the effective value of the index, or overwhelm the real index differential via gain guiding (imaginary part of the index differential).
In three-dimensional waveguides, the geometry is generally more complicated and a simple relation such as Eq. 1.1 cannot be universally obtained. Nonetheless, it is approximately true that, for a symmetric, more or less rectangular wave-guide geometry, a relationship similar to Eq. 1.1 applies independently for each of the lateral and transverse directions with characteristic waveguide dimensions WL and WT and with suitably defined index differentials xcex94nL and xcex94nT. (More precisely, Eq. 1.1 holds when the wave-guide is symmetric in both directions and the wave equation is separable in the two variables).
Very small index differentials can be obtained in the lateral direction using a stripe waveguide structure, such as a ridge waveguide, where the index differential is determined by an effective index approximation. This mathematical approximation is the consequence of a geometric structure that is achieved by constructing a lateral stripe (usually by etching grooves or entirely removing material on either lateral side of the stripe that extends on the optical axis of the device) into a heterostructure of stacked layers of varying index. In this manner, it is possible to produce sufficiently small, controllable index differentials so that truly single spatial mode operation in the lateral direction can be obtained for mode widths up to 2 to 4 micrometers.
In the transverse direction, however, the index variations are tied to band gap variations that must have minimal differentials to achieve appropriate carrier confinement within the gain region of the structure. Some have proposed designs that have modal widths up to about 2 micrometers in the transverse directionxe2x80x94if the gain region is positioned near the null of the next higher mode, the structure can effectively support only the lowest order spatial mode. The next mode, though a proper mode of the structure, has very low gain. With this strategy, the right-hand side of Eq. 1.1 may be multiplied by an additional factor of 22, to allow for the next propagating (but low-gain) mode. Then for a 2-micrometer transverse width, the maximum index differential can be as large as about 0.1, a value compatible with adequate carrier confinement.
Nonetheless, even with these efforts, mode size matching between the single mode fiber and the edge-emitting stripe waveguide semiconductor chip is still suboptimal.
In order to go beyond the limitations of conventional structures, as described above, it is necessary to use modified waveguides that support a few higher-order spatial modes, or may even be highly overmoded waveguides. The flexibility, afforded by this approach, in the geometry of the waveguide structure allows for better optimization of the mode shape and can yield less critical dimensional control.
The present invention concerns a design that uses a single-mode fiber angle-coupled to a semiconductor wave-guide medium with optical gain. This design is particularly simple and relevant to optical fiber systems, but it may be generalized to include other implementations as well, in which other single-mode filters are employed. A core realization behind the invention surrounds the fact that external cavity systems with tilted facet semiconductor active devices are generally thought to be impracticable because large facet angles, ie., greater than 7 degrees, are required to prevent self-oscillation between the facets of the semiconductor waveguide medium. At such angles, the resulting coupling efficiency is usually unacceptably low. However, when chip designs are utilized that support lowest order modes of sizes greater than about 5 micrometers (xcexcm), much smaller facet angles can be employed while still avoiding self-oscillation. More specifically, according to some aspects of the invention, facet angles of less than 4-5 degrees are utilized.
The present invention utilizes a much larger mode size thereby reducing the angle required to avoid self-oscillation. This small angle in combination with the large mode size yields high coupling efficiency (i.e, greater than 80%) without intervening optics such as microoptics or fiber lenses. As a result, efficient laser operation is achieved with the single mode fiber providing intracavity spatial mode filtering, which ensures that the laser power is carried predominantly in the large mode of the semiconductor waveguide.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.